Maybe I'll try the dead center bullets with the MMP sabots instead of the sabots that they come with next time. What do you think ?
I think that's a great idea.
Rifles can show a definite preference for certain brands and styles of sabots.
And MMP has several different sabot designs which may or may not help.
But when testing, it's recommended to only make one change at a time with the sabots, bullets, powder, primers etc...
Also, if the initial testing is performed at a slightly closer range, then there is less of a chance of sighting and shooter error showing up in the target results.
Try to keep the barrel temperature and the cleaning regimen the same between shots if possible.
Muzzle loaders are notorious for having occasional flyers. They can be minimized but not usually eliminated altogether. Some believe that it's due to some 209 primers being too hot or powder inconsistencies. The new fluffy Blackhorn 209 powder is expensive but is considered to shoot very
clean and consistently. So that's a long range option to consider.